I find just the opposite of what Lefty said about the thumb on top curing the 'wow' on the back cast. He ain't Borger but Steve Rajeff cured me of throwing the line in a curve on the back cast as you mention by a simple grip change to a semi-V on top grip.

Grip an imaginary rod right now with a thumb up grip and make an imaginary back cast. The most natural way to make the stop is to subconsciously twist the grip slightly so that your thumb turns inward towards you.

Now start the cast with a 'V' formed by your thumb and forefinger (V Grip as it was formerly called) When you make the stop because your hand is already rotated you won't tend to twist the rod as much and will cure some of the 'wow'.

Take a look at the video below. The caster pronates his wrist and palm as he moves the rod for the backcast and then supinates it back for the forward cast. You can see the reel turn out more as he brings the rod back. Is this what you mean?

Some casters will start with the thumb on top grip but when they stop on the backcast, the thumb has rotated in and the reel out so that they are really stoping with more of a V grip. I watch the position of the reel to indicate wrist pronation or supination during the cast.